Three harvest areas in St. Bernard, Plaquemines and parts of Lafourche and Jefferson parishes have been closed to oyster fishing as a precaution in the past two weeks. But the root of the stomach flu outbreak remains a mystery to state health officials and those in the industry.

The disease, Norovirus, is a more common and much less serious oyster-related sickness than the bacterial vibrio vulnificus disease that prompted the Food and Drug Administration to flirt with restrictions on raw oysters last fall. But the symptoms of Norovirus - fever, chills, aches, nausea, and diarrhea that can last up to two days - are not to be underestimated.

"It's certainly not a bad virus, but it's one that gets everybody's attention," said Jimmy Guidry, state health officer with the Louisiana Department of Health and Hospitals.

As of last week, 38 cases of Norovirus in Mississippi and the New Orleans area were tied to oysters from the three separate zones in Louisiana waters. Eleven people got sick near Pascagoula after eating oysters at a seafood-related conference at the Grand Bay National Estuarine Research Reserve; another 13 got sick at a wedding in New Orleans.

Although DHH has confirmed that everyone who was sick had eaten oysters, Guidry said it's still unclear whether the problem was due to contamination in the water where oysters were harvested or if someone handling them passed on the virus. Norovirus is highly contagious, and can be transferred from someone who has the virus onto the raw shellfish, or to another person through human contact.

That makes pinpointing the source of the disease particularly challenging. DHH has tested the water in the three zones for fecal coliform, often present in sewage and human waste, but found nothing. Sometimes the virus can get into the oysters if boats dump waste overboard in a harvest area.

Unlike vibrio vulnificus bacteria, which can be reduced through refrigeration or largely eliminated by post-harvest treatment methods, Norovirus in oysters is difficult to track or prevent.

The Plaquemines beds will remain closed through Thursday and the St. Bernard grounds through next week. The oyster grounds near Grand Isle in Lafourche and Jefferson parishes are still under investigation, because DHH officials believe the problem may be confined to one specific harvest area.

Industry officials say the closures have not markedly affected the supply of oysters. There are still vast areas of the state that remain open to fishing, and no new cases have been reported in the past week.

Sherry Koptish, a manager at Felix's Restaurant and Oyster Bar in the French Quarter, said a few people have asked about the virus but that the bar has been full all week.

John Tesvich, a co-owner of AmeriPure Oysters in Franklin who chairs the state's oyster task force, said the supply is still brisk but that the number of closures has been unusual this year.

"It happens annually in some areas of the country, and unfortunately we've had more than our share this year," Tesvich said. "We expect everything to be back to normal in the next couple weeks. This is the way it's supposed to work. Industry would rather stop harvesting if there's any indications of a problem with the quality of the product."

What's most confounding is the geographic expanse of the closures, in parts of the state that are not connected. Some believe the series of extreme weather events this winter, with numerous fronts bringing lots of rainfall during low tides, may have circulated more runoff from the land into the water.

In the 1990s there was a series of Norovirus closures in December that hit the industry hard around Christmas and New Year's, a popular time for the salty delicacy. Although it is later in the season, industry observers say that any time news of illnesses breaks there will be impacts on consumer attitudes.

"Easter time is not the peak for oyster consumption, but I wouldn't say it's the bottom of the barrel, either," said Patrick Banks, who manages the state's oyster program at the Louisiana Department of Wildlife and Fisheries.